When producing cellulose-based products, a roll press is frequently used for washing and dewatering the cellulose-based pulp. The pulp is passed between two cooperating press rolls installed in a vat included in a casing. The press rolls have a perforated outer surface, a so called mantle surface, whereby the outer surface is permeable to liquid pressed out of the pulp, and the pulp is pressed in the roll nip, or the press nip, between the press rolls, whereby liquid is pressed out of the pulp. The roll press also includes one or more washing zones prior to the press nip. One example of such a roll press is disclosed in U.S. Pat. No. 3,980,518, where the central axes of the press rolls lie in substantially the same horizontal plane. A gap is formed between the vat and the outer surface of the press rolls, and the pulp is fed in the gap in the direction of rotation of the press rolls and through the press nip.
The dimensions and weight of the vat and the casing, which includes the gable constructions of the apparatus, must be great so that the casing can withstand the great forces acting on the vat, which forces originate from the internal overpressure produced within the vat during operation.
The vat in which the two press rolls are installed and partially enclosed limits accessibility to the press roll, especially if the vat encloses a large portion of the circumference of the perforated outer surface of the press roll. Accessibility to the press roll is necessary for performing maintenance work, cleaning the press roll and the press nip, e.g. clearing plugs and removing pulp jammed on the perforated outer surface or in proximity to the press roll, or for changing press rolls. One way of providing accessibility to the press rolls and the press nip is to provide the roll press with pivotable vat segments, as disclosed in U.S. Pat. No. 3,980,518.
Pivotable vat segments are also disclosed in European Patent No. 1,035,250 which describes apparatus for washing and dewatering pulp comprising two filter drums arranged to rotate in opposite directions to create a press nip. A gap is formed between the vat and the outer surface of the filter drums, in which gap the pulp is fed while being pressed and washed. The filter drums are installed in a vat which comprises two pivotable upper vat segments for providing access to the filter drums. In European Patent No. 1,035,250, the pivotable vat segments are pivotally mounted to different beams which are mounted at respective longitudinal sides of the apparatus. These two beams are anchored to the ends of the casing or frame of the apparatus. The dimensions and weight of the beams, the pivotable vat segments, and the gable constructions of the apparatus, respectively, must be great so that the casing can withstand the great forces acting on the pivotable vat segments, which forces originate from the internal overpressure produced within the vat during operation.
Large and heavy constructions are expensive to produce, and difficult and expensive to transport and install.
One object of the present invention is thus to reduce the weight of an apparatus for washing and dewatering pulp.